Add the Proxies you want to allow into that list, otherwise Guardian will block nearly everything it sees that might be a proxy. Since this just started happening, I'd check with your host and see if they added any extra security or made any changes that would make Guardian misidentify regular IPs as being from a proxy. In the meantime, turning off Proxy checking or turning off Guardian should work to let you and your members in.

I'm thinking that since this happened to a forum that had been running fine, it's not a bug but something that happened on the server-level - a change in some sort of security protocol on your host.

If you only have one solution to a problem you're not trying hard enough!